How to diagnose and fix the HV090 fdw_invalid_string_length_or_buffer_length error code in Postgres. 

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The HV090: fdw_invalid_string_length_or_buffer_length error code in PostgreSQL indicates a problem related to string length or buffer length when using a Foreign Data Wrapper (FDW). This error typically occurs when the length of a string being passed to or from the foreign server does not match the expected or allocated length.

Here are some steps to diagnose and fix this error:

  1. Check the Data Types and Modifiers:
    Ensure that the data types and length modifiers in the foreign table definition match those of the corresponding columns in the external data source. For example, if a column in the foreign data source is defined as VARCHAR(100), the corresponding column in the PostgreSQL foreign table should also be defined with the same or a compatible type and length.
   CREATE FOREIGN TABLE foreign_table (
     id integer,
     string_column varchar(100) -- Ensure this matches the external source
   ) SERVER foreign_server;
  1. Review String Manipulation Functions:
    If you are using functions that manipulate strings, such as substring, concat, or any custom functions, ensure that they do not result in strings that exceed the expected length.
   SELECT substring(string_column FROM 1 FOR 100) FROM foreign_table;
  1. Examine FDW Options:
    Some FDWs allow for specifying options that affect how strings are handled. Review the FDW documentation to check if there are any options related to string or buffer length that need to be set or adjusted.
  2. Check for Data Truncation:
    Data truncation can occur if the string data being inserted or updated in the foreign table is longer than the column length defined in the external data source. Ensure that the string data does not exceed the maximum length allowed.
   INSERT INTO foreign_table (string_column) VALUES ('string with length less than or equal to 100 characters');
  1. Adjust Application Code:
    If the error occurs during an application operation, review the code to ensure that it correctly handles string lengths when interacting with the foreign table. This might involve adding checks or validations before attempting to insert or update data.
  2. Review FDW and Database Encoding:
    Encoding differences between PostgreSQL and the external data source can lead to string length discrepancies. Make sure the character encoding is compatible and correctly configured on both sides.
  3. Use Debugging Functions:
    Utilize PostgreSQL’s debugging functions to log and examine the exact queries and data being passed to the foreign server. This can help identify where the mismatch in string or buffer length is occurring.
  4. Consult FDW Documentation:
    Different FDWs may have specific limitations or behaviors. Consult the FDW’s documentation for any known issues or additional troubleshooting steps related to string and buffer lengths.

By carefully checking the data types, lengths, and application logic, you can usually identify and correct the cause of the HV090 error. Remember that precise solutions may vary depending on the specific FDW in use and the external data source’s requirements.

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